1. Field of the Invention
The present invention relates generally to the field of wet chemical baths used in processes for treating workpieces, for example, plating processes. In particular, the present invention relates to automatic analysis and control of concentrations of chemicals in bath solutions so that the concentration of all bath chemicals remains constant throughout the entire bath process.
2. Description of the Prior Art
Chemical baths are solutions, normally but not necessarily aqueous, which contain one or more chemicals, hereinafter called chemical species. These chemical species exist in certain concentrations in the solution. Some of these species interact or chemically react with a material, called a workpiece, which is placed in the bath. Examples of such chemical baths or "wet processes" are electroplating, electroless deposition, chemical and electrochemical etching, pickling, adhesion promotion, seeding (deposition of a thin catalytic layer on a workpiece), photographic and lithographic developing, and rinsing. Most of these baths interact with the workpiece to produce changes in the workpiece surface, e.g., addition of a film on the workpiece surface or removal of some of all of the initial workpiece surface.
While the chemical bath and the workpiece are interacting, both the bath and the workpiece change. Chemical species may leave the bath, i.e., the species is depleted from the bath, to deposit or plate on to the workpiece. Alternatively, a chemical species may be removed (e.g. etched) from the workpiece and enter, or accumulate in the bath. Other ways chemical species deplete or accumulate in a bath are discussed below. As the bath changes due to the accumulation and/or depletion of these species, the bath is said to "age."
In most industries, including the electronics industry, a majority of such processes are operated in the batch mode, i.e., one bath solution is used at a time. These batch processes often have a simple control scheme that replenishes one or even several of the depleted or consumed chemical species. Replenishment is used to keep the bath concentration of the depleted species from decreasing. In other words, the control scheme tries to prevent the bath from aging (changing) by replacing the depleted amounts of some consumed species.
Because the prior art has not accounted for all the species that deplete or accumulate in these wet processes, the baths controlled by the prior art age both as they are used and as time passes. As a result, the performance (i.e., the affect on the workpiece) of prior art baths changes and finally degrades to an unsatisfactory point. Furthermore, as the bath is degrading, the produced workpieces become more nonuniform and eventually fall outside of their required specifications. Ultimately, these baths are discarded and replaced with a fresh bath solution.
The prior art has various approaches to monitor and/or to control chemical baths but teaches substantially the same general method.
First one or more measurements of the bath are made. These measurements, taken manually or automatically, can be "off-line" measurements of a bath sample or "in-line" or "on-line" measurements taken as the bath is working.
The sample then may be treated in some manner to place the sample in a condition for the next step, i.e., analysis. This treatment might include the addition of solvents, water, acid, or metal ion to the sample.
The treated sample may be analyzed for one or a plurality of chemical species using one step or a sequence of steps. Analyzers, using chromatography, titration, spectrophotometry, electrochemical sensors or other equipment or techniques, develop signals that relate to the actual concentrations of each analyzed chemical species in the bath. Note that analysis and signal development can be done for other bath characteristics, called parameters, which include bath temperature, pH, level (volume), specific gravity and plating (process service) electrical current.
The actual chemical species concentration signals or bath parameter signals are then compared to their respective target value. The amount of species replenishment or other corrective action depends on the difference, or error, between the measured and target values.
The prior art teaches a number of corrective actions that are based on comparison between a measured value and a target value. One process takes action when established limits are exceeded. Some processes set up a ratio or a certain "quality index" as a reference. Still another process uses an "adjustable rated value" as a reference. An additional process takes action when bath components fall outside a standard concentration range. A different process adds make-up solutions when the bath solution "state" compared to a "preselected state" generates an error signal.
Based on the error value, types of dispensing apparatus, such as control valves and metering pumps, dispense the appropriate materials into the bath. An apparatus may also indicate and record concentrations of the chemical species or other relevant parameters of the bath.
The prior art discloses a computer or some other similar equipment used to coordinate and/or supervise scan, data logging, and system control functions for these various processes.